Compounds with Large Lateral Substituents

A particularly striking example of shielded lateral substituents occms in the naphthoic acids (compounds 84-87). Even large lateral substituents cause an increase in clearing point because of the efficient filling of space which enhances intermolecular attractions. The smectic tendency of the naphthoic acids (85-87) with the lateral substituents is very high. 

Whereas compounds with large flexible lateral substituents, assuming that the lateral chains are oriented nearly parallel to the basic molecule, may be considered as variants of the classical rod-like molecules, compounds with lateral ring-containing substituents lead to completely new concepts of mesogens. 

On their own polyethers have had limited use in supramolecular chemistry. Low molecular mass compounds have been shown to bind a number of metal ions, notably those in the lanthanide series that can accommodate ligands with large numbers of oxygen donor atoms. One of the main problems is the lack of diversity in the compounds functional groups which limits the range of their ligating opportunities. Where polyethers have been highly successful is as substituents to other molecules, such as calixarenes, and in their cyclic forms as the crown ethers, which will be described later. 

The use of latoal substituents in liquid crystals has proved to be very important, initially in nematic material and lata- in smectic C matoials. Clearly, anything that sticks off the side of a rod-like molecule will tend to reduce the liquid crystal phase stability, and genmlly the larg the lateral substituent the greater the reduction in liquid crystal phase stability. Usually, the smectic phase stability is much mmre affected than that of the nematic phas especially by larger substitumts because of the obvious reduction in lateral attractions, but increased lateral attractions associated with polar substituents cause a smaller reduction in smectic phase stability (see compounds 52-57) [46]. 

Esters are very common liquid crystal compounds, and lateral fluoro substitution has provided some interesting modifications to melting points, transition temperatures, mesophase morpholo and physical properties. Compounds such as 66 [26] allow for the gen ation of high positive dielectric anisotropy, but the fluoro substituent has caused a large reduction in the nematic phase stability when compared with compound 20, due to the reduction in antiparallel correlations. However, the fluoro substituent in compound 67 [26] is somewhat shielded by the zig-zag nature of the ester linkage hence the nematic-isotropic transition temperature is identical to that of compound 20. The lateral fluoro substituent in compound 68 [51] is not as shielded by the zig-zag structure and the nematic-isotropic transition temperature is much reduced, but not to the same extent as for compound 66. 

Most of the classical dienes are hydrocarbons, like cyclopentadiene. butadiene, anthracene, 9,10-dimethylanthracene, isoprene, 2,3-dimethylbutadiene. For dienes of this kind it is generally true that electrophilic dienophiles are the most reactive, and actually acrylic derivatives, maleic anhydride, p-benzo-quinone and similar compounds have found a large use as dienophiles both for preparative purposes and for kinetic studies. The latter demonstrated quantitatively the importance of electronegative groups on the dienophiles, and conversely of electron-releasing substituents on the diene, in order to accelerate such type of Diels-Alder reaction. It was also realised later that Diels-Alder additions with inverse electron demand , that is between electrophilic dienes and nucleophilic dienophiles, do occur . ... 

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